[vert.glsl]

#version 120

attribute vec2 in_Position;

varying vec4 pass_Position;

void main(void)
{
    pass_Position = vec4(0.5 * in_Position + 0.5, 0.0, 1.0);
    gl_Position = vec4(in_Position, 0.5, 1.0);
}

[frag.glsl]

#version 120

uniform sampler2D u_Texture;

varying vec4 pass_Position;

float rand(in vec2 p, in float v)
{
    return fract(v * sin(dot(p, vec2(1298.9837, 7823.33145))));
}

float point2segment(vec2 p1, vec2 p2, vec2 a)
{
    float l2 = dot(p2 - p1, p2 - p1);
    if (l2 == 0.0)
        return distance(a, p1);
    float t = dot(a - p1, p2 - p1) / l2;
    if (t < 0.0)
        return distance(a, p1);
    else if (t > 1.0)
        return distance(a, p2);
    vec2 proj = p1 + t * (p2 - p1);
    return distance(a, proj);
}


void main(void)
{
    float width = 800.0;
    float height = 600.0;
    float line_width = 2.0;

    vec2 t = pass_Position.xy;
    vec2 tc1 = floor(t * 128.0) / 128.0;
    vec2 tc2 = tc1 + vec2(1.0, 1.0) / 128.0;
    vec2 tc0 = tc1 - vec2(1.0, 1.0) / 128.0;
    vec2 tc3 = tc2 + vec2(1.0, 1.0) / 128.0;

    vec4 c;
    c[0] = texture2D(u_Texture, tc0).x;
    c[1] = texture2D(u_Texture, tc1).x;
    c[2] = texture2D(u_Texture, tc2).x;
    c[3] = texture2D(u_Texture, tc3).x;

    /* Artificially compress in Y */
    c *= 0.3;

    vec2 p0 = vec2(tc0.x * width, c[0] * height);
    vec2 p1 = vec2(tc1.x * width, c[1] * height);
    vec2 p2 = vec2(tc2.x * width, c[2] * height);
    vec2 p3 = vec2(tc3.x * width, c[3] * height);
    vec2 a = vec2(t.x * width, t.y * height);

    float d0 = point2segment(p0, p1, a);
    float d1 = point2segment(p1, p2, a);
    float d2 = point2segment(p2, p3, a);

    float d = clamp(line_width - min(min(d0, d1), d2), 0.0, 1.0);

    gl_FragColor = vec4(t.y, d, d * 0.3, 1.0);
}